532 L. H. GRAY 



time, however, the problem of protecting personnel was increased. 

 Moreover, the mean energy of the neutrons, and therefore of the recoil 

 protons that ionize the tissue, has increased so that the mean linear 

 ion density of the ionizing particles has decreased, and is therefore 

 less sharply contrasted with that for the 200 kv. X rays used in ther- 

 apy (c/. Table I). 



Microorganisms that do not require a collimated beam could 

 obviously be exposed to neutron dose rates of the order of 10,000 

 equivalent roentgens per minute if placed 3 cm. from the target of a 

 60 inch cyclotron. The whole body exposure of animals is lethal at 

 relatively low doses. For mice the acute lethal dose of fast neutrons 

 has been found by Zirkle (67) to be of the order of 250 equivalent 

 roentgens and such exposures only yield a limited amount of informa- 

 tion. The exposure of tumors or individual organs in small animals 

 requires a collimated beam and presents a new problem. Aebersold 

 gives no data for beams of less than 7 cm. in cross section. Without 

 using collimation. Gray and Read {68) exposed tumors implanted in 

 the thighs of mice to neutron doses equal to four times the mean 

 body dose by placing the tumor very close to the target of their 

 (D-D) generator, so that by comparison with the tumor the rest 

 of the body was in a relatively weak radiation field. 



•1. Slow and Thermal Xeutrons 



When a neutron collides with a hydrogen nucleus energy and 

 momentum are conserved and any amount up to the full energy of 

 the neutron may be transferred to the hydrogen. It has been shown 

 experimentally by Bonner (69) for (D-D) neutrons, in agreement with 

 theory, that the neutron loses on an average half its energy at each 

 collision so that the average speed of a beam of neutrons rapidly falls 

 as the beam passes into a hydrogenous medium. In the case of water 

 or wax most neutrons are ultimately reduced in speed until their 

 energy is equal to the mean energy of thermal agitation of the me- 

 dium. They are then referred to as ''thermal neutrons." As they 

 approach this condition, however, the probability rapidly increases 

 that at any given collision the neutron and hydrogen nucleus will 

 react to form deuterium with emission of a 2.2 m.e.v. y v&y. This is 

 the mechanism by which water and wax shields offer protection 

 against fast neutrons. If it is desired to obtain high fluxes of thermal 

 neutrons, however, it is better to use heavy water or graphite, since, 



